feat: webhook payload history — store and display recent incoming payloads

Backend:
- WebhookPayloadLog model (provider_id, method, headers, body, status, extracted_fields, error_message)
- Auto-log payloads in generic_webhook() with matched/unmatched/error status
- Auto-prune beyond max_stored_payloads per provider
- Header filtering (only Content-Type, User-Agent, X-* stored; no Authorization)
- GET/DELETE /api/providers/{id}/webhook-logs endpoints
- store_payloads + max_stored_payloads in WebhookProviderConfig

Frontend:
- WebhookPayloadHistory.svelte — expandable log viewer with status badges, JSON body, headers, extracted fields
- payloadHistory flag on webhook provider descriptor
- max_stored_payloads config field (0 = disabled)
- Password confirmation field on change password modal
- i18n keys for webhook logs (en + ru)

frontend/src/routes/providers/+page.svelte

@@ -20,6 +20,7 @@
 	import { highlightFromUrl } from '$lib/highlight';
 	import { getDescriptor, buildProviderFormDefaults } from '$lib/providers';
 	import Button from '$lib/components/Button.svelte';
+	import WebhookPayloadHistory from './WebhookPayloadHistory.svelte';
 	import type { ServiceProvider } from '$lib/types';
 
 	let allProviders = $derived(providersCache.items);
@@ -263,6 +264,9 @@
 					</div>
 				</div>
 			</Card>
+			{#if provDesc?.payloadHistory && !showForm}
+				<WebhookPayloadHistory providerId={provider.id} />
+			{/if}
 		{/each}
 	</div>
 {/if}